1. Field of Invention
This invention relates to an improvement in the preparation of a Ziegler catalyst for the polymerization of olefins wherein the improvement involves a more ready control over the reduction of the catalyst component and consequently a better control over the particle size and composition of the catalyst.
2. Problem to Which Invention is Directed
TiCl.sub.3 cocrystallized with AlCl.sub.3 can be prepared by reduction of TiCl.sub.4 with aluminum alkyl halides. It is desired in the industry to prepare catalyst samples where the individual catalyst particles have diameters greater than 12 microns and where the catalyst samples have a narrow particle size distribution. These goals can be achieved by using as a reducing agent AlEt.sub.2 Cl or AlEtCl.sub.2. Any attempts to use AlEt.sub.3 lead to catalyst particles whose diameters are normally less than 3 microns. With AlEt.sub.2 Cl or AlEtCl.sub.2 as reducing agents, the aluminum chloride content in the catalyst product, TiCl.sub.3.nAlCl.sub.3, is higher than desired and may lead to catalysts of low activity. Catalysts with optimum aluminum chloride content, where n = 0.25-0.35, can be prepared using compounds such as AlEt.sub.3 or trialkyl aluminum as reducing agents. As pointed out before, trialkyl aluminum reducing agents lead to fine catalyst particles.
3. Description of Prior Art
It is well known in the prior art that olefins such as ethylene, propylene or higher olefins (as well as diolefins) can be polymerized to polymers of medium and high molecular weight by contacting them with Ziegler catalysts. Of these many Ziegler catalysts, the composition of TiCl.sub.3.nAlCl.sub.3, where n is usually about 0.33, in conjunction with a cocatalyst, such as AlEt.sub.2 Cl or AlEt.sub.3, is most commonly used. The TiCl.sub.3.nAlCl.sub.3 can be prepared by various techniques of which the following two are the ones most commonly employed:
(a) reduction of TiCl.sub.4 with aluminum PA1 (b) reduction of TiCl.sub.4 with aluminum alkyl halides
By proper manipulation of conditions in the latter reduction procedure, one can obtain catalyst samples whose particle size distribution is narrow and whose particle size (diameters) can be varied. Most commonly the reducing agent used for the reduction of TiCl.sub.4 is AlEt.sub.2 Cl as disclosed in British Pat. No. 1,139,450 assigned to Shell International Research, and South African Applications Nos. 67-7828 and 67-7829, assigned to Hercules Inc.
It is also well known in the prior art that TiCl.sub.3 catalysts such as these, when heated to temperatures above 150.degree. C, are converted from their .beta. form to the more active .alpha. or violet form. As for example in U.S. Pat. No. 2,971,925 to Winkler et al and U.S. Pat. No. 3,261,821 to Vandenberg.
By the use of different reducting agents, catalysts of different compositions are obtained. Furthermore, the use of different reducing agents in the reduction of TiCl.sub.4 under proper conditions may lead to catalyst samples with different physical properties. It is highly desirable to obtain catalyst samples with optimum catalyst compositions along with proper physical properties. It is toward this goal that this invention is directed.
The prior art on mixed alkyl reduction also includes the following:
U.S. Pat. No. 2,943,068 (Jan. 28, 1960), L. T. Eby et al, claims brown reduction product of TiCl.sub.4 and AlEt.sub.2 Cl/AlEt.sub.3 for polymerization of ethylene. The product which becomes inactive on standing may be reactivated with AlEtCl.sub.2. I claim very specific conditions to prepare a spherical reduction product which is purple for the polymerization of propylene.
U.S. Pat. No. 2,962,491 (Nov. 28, 1960), Mertzweiler, claims partial reduction of TiCl.sub.4 with AlEt.sub.2 Cl and AlEtCl.sub.2. The mixture AlEt.sub.2 Cl/AlEtCl.sub.2 is entirely different and will give different results than AlEt.sub.2 Cl/AlEt.sub.3.
U.S. Pat. No. 3,108,973 (Oct. 29, 1963), Vandenberg, claims TiCl.sub.4 reduced by AlEt.sub.2 Cl or mixture of AlEt.sub.2 Cl/AlEtCl.sub.2 and activated in polymerization with mixture of AlEt.sub.2 Cl/AlEt.sub.3. Same argument as above applies. The mixture of AlEt.sub.2 Cl/AlEt.sub.3 used in polymerization would be expected to give different results than when used in reduction.